In conventional borehole electromagnetic investigation techniques, a sonde having one or more transmitters and receivers is lowered into a borehole and logged back to the surface while making measurements. Because the transmitters and receivers are located in the same sonde body, their relative position (distance and alignment) is fixed and variations in measurements can be attributed to variations in the formation surrounding the borehole rather than to relative movement between the transmitters and receivers. This condition even applies to logging-while-drilling tools since the relative positions of the transmitter and receiver are fixed there also.
The depth of investigation of an electromagnetic technique for investigating underground formations is typically dependent upon the separation of the transmitter and receiver. In conventional electromagnetic measurements, the depth of investigation is of the order of 0.1 to 10 meters. For very deep measurements or cross-well measurements, which might have a depth of investigation of 10 to more than 100 meters, it is impractical to locate the transmitter and receiver in the same rigid sonde or tool string due to the large transmitter-receiver spacing required or the need to separate the functions between wells. Once the transmitter and receiver are separated, even when in the same well and connected by wireline cable, there is a possibility of relative movement in their alignment which will have an effect on the response of the tool.
This problem is also encountered in other deep measurement techniques such as cross-well seismic measurements which require that the relative position of transmitters and receivers be reasonably well known. In cross-well seismic techniques, both borehole trajectories are accurately mapped with a triaxial accelerometer to determine the vector-valued up-down direction of gravity and a triaxial magnetometer to determine the vector-valued north-south direction of the earth magnetic field. These measurements usually are wireline based. They are integrated along the wellbore trajectory and serve to determine the absolute position of each point with respect to the (x,y,z)-coordinates of a reference point at the surface of each well. The vector-valued distance between the two well-reference points at the surface are measured using standard geodesic surveying techniques. Finally, the three vector-valued distances are combined to determine the vector-valued distance, namely the relative position between transmitters and receivers. In addition to providing the absolute position of each point towards a surface reference, the survey also provides an absolute orientation of any oriented measurement with respect to up-down (gravity) and north-south (magnetism). The uncertainty in the transmitter and receiver positions from these surveys can be entered in the survey inversion processes as unknowns in addition to the unknown formation parameters (compressional and/or shear slowness for seismic surveys, electric conductivity--and maybe magnetic permeability--for electric surveys).
B. R. Spies, Survey Design Considerations for Cross-Well Electromagnetics, 1992 Annual SEG Meeting, New Orleans, observes that very low frequency electromagnetic signals contain no information on the conductivity of the formation and proposes that such signals could be used to calibrate the field system and possibly correct for errors in the source-receiver geometry.
It is an object of the present invention to provide a technique for determining the relative vector-valued distance and orientation between the transmitters and receivers without reference to the surrounding formation.